Combination variable area and wiggle-line display of seismic signals

ABSTRACT

Seismograms are represented in a combined wiggle trace and variable area display for side-by-side presentation in a seismic time or depth display. Seismic wiggle traces are sine wave representations of earth motion detected by a seismometer and recorded with respect to time. The variable area display portion is presented as a truncated part of each positive going half of each sine wave forming such a seismic trace. The truncated part is filled or darkened as by exposure of a film to a cathode-ray beam under the control of a sawtooth wave. A particular advantage of the present arrangement is that the sawtooth wave is inverted so that the inherently ragged portion at the beginning of each sawtooth wave is blanked out and only the upper, clean part of each line recorded. A common peak level of the sawtooth wave is then positioned on the zero axis so that the positive portion of each half cycle of the seismic signal is cleanly filled. The resulting display is sharper and cleaner than previously known variable area-wiggle trace displays.

United States Patent LINE DISPLAY OF SEISMIC SIGNALS PrimaryExaminer-Joseph W. l-lartary AttorneysA. L. Snow, F. E. Johnston, R. L.Freeland, Jr. and

H. D. Messner ABSTRACT: Seismograms are represented in a combined wiggletrace and variable area display for side-by-side presentation in aseismic time or depth display. Seismic wiggle traces are sine waverepresentations of earth motion detected by a seismometer and recordedwith respect to time. The variable area display portion is presented asa truncated part of each 6 Claims, 12 Drawing Figs.

positive going half of each sine wave forming such a seismic [52] US.CL: 346/1, trace The truncated pan is filled or darkened as by exposure346/110 340/155 of a film to a cathode-ray beam under the control of asaw- [5l] Int. Cl G0lv 1/32 tooth wave A particular advantage f thepresent arrahge [50] held of Search 346/ 1,33 mam is that he Sawtoothwave is inverted so that the 1 [08;340/155 DS herently ragged portion atthe beginning of each sawtooth [56] defences Cited wave is blanked outand only the upper, clean part of each line recorded. A common peaklevel of the sawtooth wave 18 then UNITED STATES PATENTS positioned onthe zero axis so that the positive portion of each 3,158,433 1 1H964AleXandel' 6t 81 /1 1 half cycle of the seismic signal is cleanlyfilled. The resulting 3,173,743 1965 WeiSSenSteiflef- 4 display issharper and cleaner than previously known variable 3,349,407 l0/l967Hefer et al. 346/1 10 area-wiggle trace displays.

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ilrIr(((((/// dam COMBINATION VARIABLE AREA AND WIGGLE-LINE DISPLAY OFSEISMIC SIGNALS FIELD OF INVENTION The present invention relates todisplay of seismic signals by photographically recording theinstantaneous image of the beam of a cathode-ray tube in response to anapplied seismic signal. More particularly, it relates to a method of andapparatus for accurately controlling a cathode-ray beam to record acombined wiggle trace and variable area representation of the amplitudeand frequency of a seismic signal.

OBJECT OF THE INVENTION It is a particular object of the invention topresent a cleaner combined wiggle trace and variable area display thanhas heretofore been possible, by more accurately positioning the cathoderay beam during recording of the variable area portion of each combinedseismic signal trace. By using only the peaks of a sawtooth waveform tofill one-half of each cycle of the wiggle trace jiggle, or roughness, inthe relatively unstable bottom portion, or trough, of each sawtooth waveis eliminated from the display. In a preferred form of apparatus tocarry out the method, the peaks of the sawtooth waves, used to fill thepositive half of each cycle of seismic signal trace, are positioned onthe zero crossing axis of the wiggle trace portion of the display andthe bottom of each sawtooth waveform is arbitrarily lopped off ortruncated. In this way the individual traces of the seismic time section(made up of many side-by-side traces) can be placed closer together.Such denser packing of adjacent traces better preserves the visualcontinuity of adjacent troughs of each seismic signal trace.

BACKGROUND OF THE INVENTION In present day seismograph exploration, itis customary to record a seismic disturbance generated by an explosionor an impact of the earth at a plurality of geophones. Sonic velocitydiscontinuities in the earth as between earth formations of differentacoustical impedance create reflections that are then detected by thegeophones spacedalong a line to form a series of spaced stations. Eachstation of the line includes a geophone, or several electricallyconnected together, that generates an oscillating electrical signalrecordable as a single seismic trace. Usually 24 or more traces arerecorded simultaneously. In present day seismic exploration, thesetraces are individually filtered and adjusted in phase or amplitude toenhance the information content and to reject the noise components ormisalignments that normally occur in field recording. The final displayof each individual geophone, whether from a raw, or treated, fieldrecord or from an office calculated or reconstructed trace, is thenrecorded in a side-by-side relationship to its adjacent field geophoneposition on a single graph, called a seismic time section. A pluralityof seismic time sections made up of the recorded plurality of traces arethen usually combined to form a seismic profile, generally representingseveral miles of subsurface coverage along a line of survey. Becauseseveral hundred traces are required for a complete seismic profile, andin general geologic structure is deduced from a plurality of closelyadjacent seismograph traces, it is important that the traces be asclosely spaced as possible, but that each individual trace be as cleanand clear as the recording process permits.

It has been common heretofore to use various combinations of wiggletrace, variable area or variable density displays to produce seismictime sections, and in turn seismic profiles. One convenient way torecord a seismic profile is to photograph the instantaneous defiectionof a cathode ray beam on the face plate of a cathode-ray tube. Relativemotion between the spot on the cathode-ray tube screen and photographicfilm produces a sinusoidal curve. It is most customary to rotate a drumwith a photographic negative wrapped around it past a focused image ofthe beam striking the screen of a fixed cathode-ray tube. Thecathode-ray beam is deflected only horizontally or laterally torepresent amplitude of an inrangement is that the drum may rotatecontinuously, but the I carriage of the cathode ray tube need movetransversely to the drum surface only at the end of each revolution andthen only the desired separation width of one seismic trace relative toits neighbor traces on the record. With a straight wiggle-trace display,the amplitudes of the positive and negative going portions of each wavefrequently are sufficiently high that there is serious overlap betweenadjacent traces. To avoid such overlap, variable area and variabledensity displays of a single line of a given width are frequently usedto represent amplitude and frequency variations of each cycle of theseismic signal. However, such variable area or variable density displaysare difficult for the seismologist to read and analyze. Accordingly, itis more desirable to present a wiggle trace and then emphasize certainparts by superimposing a variable area display on the wiggle trace.

In previously known variable area and wiggle trace displays, the linesfilling the area under one or both sides of the wiggle trace excursionsrelative to the zero axis have been fuzzy. These lines are formed byoscillating the cathode-ray beam at a frequency substantially higherthan the wiggle trace wave. When this wave is of relatively lowamplitude, the roughness, or fuzzying of the lines is particularlyobjectionable.

SUMMARY OF INVENTION 1 have found that such roughness in filling thearea under the wiggle trace is primarily due to jitter in the initialpart of the sawtooth wave used to generate these lines. When this ini-'tial part is displayed with its base near the zero-crossing axis of thewiggle trace, and the top of the sawtooth wave is cut off-by crossingthe wiggle trace or an arbitrary width line. such as the variableenvelope of the clipped sine wave peak, the variable area portion may betoo ragged to recognize as a signal from the recorded variable area. Inaccordance with the present invention, a sawtooth wave is inverted andits total amplitude made greater than the expected extrema of thesinusoidal voltage traces representing the seismic signal. The top orpeak'of each sawtooth wave (as defined by the point of transition from arelatively slow amplitude change to a relatively fast amplitude changeas by the retrace between each wave in the sawtooth signal) are set at apredetermined level relative to the mean line or zero-crossing axis ofthe sine wave. Desirably the points are aligned to lie on thezero-crossing axis. However, in other forms of variable are display,they may lie above or below this line. Additionally, the sawtooth waveis blanked during the time it is near the base line and until it crossesa certain predetermined point, such as the envelope of the clipped sinewave peak, on the wiggle trace.

In a preferred form of apparatus for carrying out th method of thepresent invention, the photographic film upon which the seismic sectionis recorded is moved in the time or depth direction by a drum rotatingpast the beam image on the face of a cathode-ray tube. The face of thecathode-ray tube is then photographed as the cathode ray beam oscillatestrans; versely to the direction of movement of the paper.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a block diagram and schematicrepresentation of a preferred form of apparatus for carrying out themethod of the present invention.

FIG. 2 is a detailed wiring diagram illustrating one form of electroniccircuitry used in the arrangement of FIG. I.

FIG. 3 is a set of waveform diagrams illustrating in greater detail theoutputs of various parts of the circuits shown in FIG: 2.

FIG. 4 is a set of diagrams illustrating the display of an idealizedseismic trace generated by the prior art and as modified in accordancewith this invention by various parts of the apparatus of FIGS. 1 and 2.

FIG. 4A indicates a sinusoidal wave of varying amplitude and frequencyrepresenting a seismic trace.

FIG. 48 indicates application of a prior art display to the trace ofFIG. 4A.

FIG. 4C indicates a normal sawtooth wave generated during each positivegoing half-cycle of the trace.

FIG. 4D indicates an inverted sawtooth wave similar to FIG.

FIG. 4E is a truncated sinusoidal wave in accordance with a preferredform of the present invention.

FIG. 4F is a combination of FIG. 4D and FIG. 4E in accordance with oneform of the present invention.

FIG. 4G is an alternative form of variable area presentation of thepresent invention.

FIG. 4H is still another form of signal representation using thisinvention.

FIG. 5 is a portion of an actual seismic profile recorded in accordancewith the preferred form of display using the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 illustrates one form ofapparatus for carrying out the method of plotting a combined wiggletrace and variable area representation of a seismic trace to develop aseismic time section and a seismic profile representing several miles ofsubsurface coverage.

As is well understood in the art of seismic reflection or refractionprospecting, a record of a plurality of geophones is generated byrecording electrical signals representing the sinusoidal movement of theearth in response to the impact and reflection or refraction of sonicenergy by acoustic discontinuities in the earth layering. The geophonesare spaced apart generally along a single line and the signals in moderntechnology are recorded on magnetic tape. The signals are in the form ofdigital impulses or an electrical analog of the geophone movement. Inthe present embodiment the field procedure is not illustrated and therecording begins with a magnetic tape 10, illustrated as being mountedupon rotating drum l2, driven by motor 14. The invisible magneticrecords are shown as traces 16 on record I0. For purpose of thisrepresentation, an individual pickup 17 is moved from trace to trace.Such movement is normally automatic. It is synchronized with movement ofthe final recording paper. This is shown schematically by the positionof the electron beam 25 on face 29 of cathode-ray tube 19 onphotographic paper 21 mounted on drum 23. Rotation of drum 23 is insynchronism with the drive of drum 12. Electron beam 25 of tube 19 isfocused on the face of photographic paper 21 by a focusing systemindicated as lens 27 which directs the light from face 29 of tube 19onto a given area of film 21. The display traces are indicated as 31.

The playback system. including magnetic tape and drum 12, as well as thedrive of photographic drum 23, relative to cathode-ray tube 19, is wellknown in the art; the present arrangement illustrates one form by whichthe method of the present invention may be used.

The output of pickup 17 is a sinusoidal curve of varying frequency andamplitude dependent upon the input seismic signal. An indication of itsvariations in frequency and amplitude is designated by the input signalwaveform 40. In FIG. I, it is applied to both peak clipper 42 and squarewave generator 44. Peak clipper 42 maintains the waveform over thetrough of each cycle of wave 40, but clips the top of the peak so thatduring peak periods the variable area display has a fixed height abovethe zero axis. The character of the original signal is thus maintainedby using only a part of the peak and the full form of the trough in eachcycle. The output of peak clipper 42 is indicated as waveform 46 showingfull troughs and clipped peaks. Suitable circuitry for performing thisfunction is described in more detail in connection with FIG. 2. A bettershowing of the waveform 46 may be seen in FIG. 4E. FIG. 4C shows theoriginal waveform designated as 40 in FIG. 1.

Square wave generator 44 squares each wave during its peak, orpositive-going, portion of wave 40. The output of generator 44 isindicated as signal 49. This wave controls the time during which thesawtooth wave is generated for use in filling the truncated part of thepeaks in the final display. A pulse source 50 supplies a suitably highfrequency, such as 20 kiloI-Iertz when the waveform of the originalsignal 40 is less than about I Hertz. The general waveform of the pulsesource is designated 52. It is applied with square waves 129 to asawtooth wave generator 54. This generator creates a generally sawtoothwaveform of the general shape shown in FIG. 4B. Designation of theoutput of generator 54 is shown as signal 56. As best seen in FIG. 4C,the general form of the sawtooth has a generally slow rise from a baseline to a peak value of known height, as indicated schematically. Theinitial part of each sawtooth is somewhat ragged due to electronictriggering and input transients that appear in a final waveform when itis reversing from a sudden vertically downward stroke to a relativelyslowly rising upward stroke. The upward stroke requires a substantiallylonger time than does the total downward stroke. It is because of thisragged initial part that the conventional sawtooth wave applied in theconventional way does not present an attractive, or useful, form ofvariable area display when used to fill a part of the sinusoidalwaveform.

To invert the waveform 56 and to assure that its total height is greaterthan the expected extrema of the peak amplitude of the input signal, aDC bias source identified as 58 is combined with the input signal 56 inan inverter and adder 60 to produce an output designated as waveform 62.The output of inverteradder 60 and peak clipper 42 are then combined inan adder and switch 64 to produce the final wavefonn designated as 66.This waveform is applied to the cathode-ray display tube I9 in two ways.The variations in amplitude are applied to the horizontal deflectionplates 68 in synchronism with the motion of drum 23 moving insynchronism with the transverse axis of the wave. At the same time it isalso desired to print on photographic film 21 only when the beam iswithin certain limits as designated by the tops of the clipped peaks. Todo this. the cathode-ray tube is blanked when the cathode-ray beam isoutside of the given width limitation. For this purpose the Schmitttrigger 70 controls the grid of electron gun 72. The general form of theoutput of Schmitt trigger 70 to electron gun 72 is indicated by waveform74.

FIG. 2 shows one form of circuit suitable for practice of the method ofthe present invention. To simplify the explanation of its operation aseismic signal wiggle-trace is indicated as a single cycle of aninitially positive-going sine wave I00. Sine wave I00 is applied toterminals I01, I02, and forms the input to the wave conversion circuitsindicated as I03 and I04. Circuit 103 controls the height of thepositive-going portion of each cycle to hold the trace within an allotedwidth span on the final record. This permits adjacent traces to befitted closely together for side-by-side display, as desired bygeophysicists who interpret such records. Circuit I04 is a square wavegenerating circuit that properly merges the sawtooth signal thatgenerates the variable area display on each positive-going half of eachwiggle trace cycle.

The output of circuit 103 is indicated by the waveform 105 at terminal106. It is generated in circuit 103 by combining a variable negativebias consisting of -15 volts and a:l0 clip level voltage. This producesa positively biased inverted signal at terminal I06. The positiveportion of this signal is fed back through diode I14 and resistor to thesumming junction of operational amplifier 110. The negative portion ofthis signal, however, causes the current to be fed back through diodeI12 only (essentially a zero resistance path), thus causing I00 percentfeedback resulting in no gain (zero volts) at terminal 106. Therefore,circuit 103 lops off (or clips) the original signal as determined by theclip level voltage which is in proportion to the spacing of the seismictraces. Circuit II7 restores the signal to its original mean level,relative to ground, by adding again the IS volts and the clip levelvoltage to the signal. Since operational amplifier 116 also inverts, theclipped peak appears as the positive portion of waveform 120 as shown atterminal 119.

In accordance with the present invention, it is the waveform 120 thatcomprises the individual seismic trace that is recorded in side-by-siderelation to other physically adjacent geophone generated traces. Thepositive half of each wave such as 120 is then filled to provide acomposite wiggle trace and variable area representation of the seismicenergy. The remainder of the circuit of FIG. 2 accurately positions thetop portion of a sawtooth wave (of relatively high frequency compared tothe seismic wave frequency) so that the peak of each sawtooth wave is onthe zero crossing axis and with the bottom portion of each sawtooth waveblanked out from its beginning to a desired level, the lopped off top ofwaveform 120.

As indicated before, the other input from terminal 101 is into circuit104. The primary function of this circuit is to form an approximatelysquare waveform having the same axis crossings as waveform 100. Circuit104 includes amplifier 123 and a pair of diodes 121 and 122 connectedfront-to-front across a resistor 124. As indicated, waveform 125,appearing as the output of circuit 104, is an approximation of thesquare wave with the crossover points being the same as those in theoriginal signal 100. Crossover Schmitt trigger 127 then takes signal125, indicated at pin 126, and through logical inverter 128 produces aninverted square wave during the positive half of the initial signal. Itswaveform at pin 129 is indicated as 130 (also shown in FIG. 3). Thissquare wave is then logically ORed by gate 131 and sent to sawtoothgenerator 134.

Generator 134 produces the sawtooth wave that fills the printed displayon each positive half cycle of the input seismic signal. As indicated atpoint 133, the output of logical-OR gate 131 is pulsed square wave 132,so phased that the sawtooth wave is generated only during each positivegoing portion of the initial signal. The sawtooth repetition rate iscontrolled by oscillator input (waveform 136) to logic amplifier 131. Inthis arrangement the repetition rate is substantially higher than thatof the fundamental waveform, 20 kiloI-Iertz as indicated by inputwavefonn 136, better seen in FIG. 3.

Amplifier 137 acts as a buffer to drive transistor 139 in circuit 134.The I5 volts source supplies condenser 138 and together with transistor139 generate in amplifier 140 the sawtooth waveform indicated as 141(also shown in FIG. 3) at outlet terminal 142. Waveform 141 is formed bya plurality of individual ramps during only the positive part of eachpositive going input signal. This output is then combined with output ofunit 117 in a combining circuit 143. As indicated, circuit 143 combinesa negative volts, sawtooth wave and the clipped sine wave to produce asan output of amplifier 144 the indicated signal at pin 145 anddesignated as signal form 146. As there seen, the combined signal isinverted and has added during the positive going portion of the originalinput signal a combination of the square-clipped sine wave with asawtooth superimposed thereon. This combined signal is then directed totwo difierent circuits, one of which controls the blanking andunblanking of the cathode-ray beam and is indicated as going to thecathode control of the cathode-ray tube through circuit 150,complementary Schmitt trigger 151, logic inverter 152, and logical-ORgate 153. Amplifier 155 of circuit 150, of course, inverts the signal146 and Schmidt trigger 151 is set to generate a train of blankingpulses which will blank the portion of the inverted sawtooth which isnot within the envelope of the clipped sine wave.

As noted above, FIG. 3 is a series of waveforms indicating one way tocombine pulses 136 and sine wave 100 and illustrates outputs of thevarious circuit elements of FIG. 2. Waveform 150A indicates the additionof the sawtooth waveform and sine wave in operating amplifier 150.During the time waveform 150A is greater than zero volts, it isrepresented by waveform 151A. produced by Schmitt trigger 151. It isalso during this time that the inverted sawtooth waveform portion offinal waveform 161 is to be unblanked. Waveform 152A, the output ofamplifier 152, is the inverse of waveform 151A, since it is desired tohave a positive blanking voltage. The input high-frequency waveform,indicated as 20 kiloI-lertz (waveform 136) is then ORed in circuit 153with waveform 152A to create waveform 153A. This operationis performedso that the flyback portions of the sawtooth waveform will be blankedeven though their voltages are greater than zero volts in waveform A.Waveform 153A is then ANDed in with the waveform at point 127A, theoutput of Schmitt trigger 127 in AND gate 154 so that blanking willoccur only during the peak, or positive portion, of each sine wave asindicated by waveform 154A. In this way only the full line portions ofthe curve designated as 161 will appear on the face of the cathode-raytube and all other portions, shown by the broken lines, are blanked bycontrol of the grid of the cathode-ray tube The values of the individualresistors in the designation of the circuit elements in one embodimentof the invention are indicated on FIG. 2.

An indication of the form of wave display contemplated by the presentinvention is illustrated in the FIGS. 4A to 4H. FIG. 4A, in general,represents a seismic trace of varying frequency and amplitude. FIG. 4Bshows the prior art applied to the waveform of FIG. 4A. As there seen, asawtooth waveform is used to fill the peaks of each positive going halfof each seismic cycle. With the peak, truncated or lopped off at a lowlevel, the jiggles in the start of each sawtooth are particularlyobjectionable. Since lopping of the peaks permits a denser display oftraces for a given time section or seismic section. the presentinvention permits a cleaner and more definite display of the seismicdata in both wiggle trace and variable area without requiring fullwaveform display. A particular advantage of the arrangement is that aseismologist may more readily understand or appreciate the correlationbetween adjacent traces (as in FIG. 5) and the information containedtherein by a denser packing and a clearer filling of the truncatedpeaks.

FIG. 4C indicates, in general, a sawtooth waveform corresponding to thegeneral waveform of FIG. 4A. FIG. 4D indicates an inversion or reversalof the waveform of FIG. 4C. FIG. 4E indicates a truncation of the peaksin accordance with the present invention and FIG. 4F shows a combinationof FIGS. 4E and 4D in accordance with this invention.

FIG. 46 is an alternative, variable area display in which the troughshave been truncated or eliminated below the zero crossing axis of thewaveform of FIG. 4A. In accordance with this invention, the tops orpeaks of each sawtooth are positioned at a known level above theexpected extrema of the seismic signal. Alternatively, as in FIG. 4H,only the peaks. that is, all parts of the signal above zero axis areshown along with a mirror image of each positive half of the signal. Insuch an arrangement, the bottoms of each peak, of course. are terminatedwhen the sawtooth reaches the zero axis of the sinusoidal curve. Thetops of the peak are truncated on both sides, as at line 200, to presenta relatively square appearing variable area trace within fixed widths.Such a display, of course, is a full variable area display withoutpreservationof waveform by the original wiggle trace.

FIG. 5 illustrates a portion of an actual seismic profile formed by aplurality of seismic traces displayed in accordance with the method ofthe present invention using a variable area-wiggle trace display mode.It will be particularly noted that the tops of the truncatedpositive-going wave on each trace exactly touches the zero or base lineof the adjacent trace. Any overlap would have resulted in a darkening ofthe lower and upper parts respectively of the neighboring traces wherethe positive amplitudes exceed the alloted space.

Various modifications and changes will occur to those skilled in the artand such changes may be made without departing from the scope of theappended claims. All such modifications coming within the scope of theclaims are intended to be included therein.

Iclaim:

l. The method of plotting a generally sinusoidal voltage trace so thatthe area between said trace and its mean line.is

shaded on one side of said means line by means of a sawtooth wave ofsubstantially higher frequency than that of said trace, comprising:

a. generating a sawtooth voltage of amplitude greater than the voltagedifference between the expected extrema of said trace and said mean lineof said trace,

b. applying at least one of the operations consisting of inversion andtranslation to said sawtooth voltage to cause its points of transitionfrom relatively slow amplitude change to relatively fast amplitudechange to be at a predetermined voltage relative to said generallysinusoidal trace, and to cause its points of transition from relativelyfast amplitude change to relatively slow amplitude change to be beyondsaid expected extrema, so that during the shading of said generallysinusoidal trace, said points of transition from relatively fastamplitude change to relatively slow amplitude change are not plotted insaid shaded area.

2. The method in accordance with claim 1 in which said predeterminedvoltage of transition points from relatively slow amplitude change torelatively fast amplitude is the mean voltage of said generallysinusoidal trace.

3. The method of plotting a single seismic trace in the form of acombined wiggle trace-variable area display which comprises generatingan electrical signal having an inverted sawtooth waveform, combining aseismic signal of generally sinusoidal waveform with a constantamplitude electrical signal and said inverted sawtooth waveform to forma composite signal, modulating at least one set of deflection plates ofa cathode-ray tube in response to said composite signal, blanking theelectron beam of said cathode-ray tube impinging on the face of saidcathode-ray tube when said inverted sawtooth wave contribution to saidcomposite signal exceeds a predetermined value, and photographicallyrecording the position of the cathode ray beam on the face of saidcathode ray tube in accordance with the time variations in theinstantaneous amplitude of said composite signal during the unblankingof said cathode-ray beam.

4. The method of recording a seismic trace as a variable area displayhaving sharp definition at low amplitudes which comprises applying to aphotographic recording medium the instantaneous output of an electronbeam displayed on the face of a cathode-ray tube, said electron beambeing under the combined control of a sawtooth waveform having afrequency substantially higher than the highest frequency of the seismicsignal to be displayed and the seismic signal waveform, the improvementcomprising inverting said sawtooth waveform, biasing said invertedwaveform to generate a waveform emphasizing substantially only the peakportion of each sawtooth wave, modulating at least a part of each cycleof said seismic signal in response to a train of said biased andinverted waves, applying said modulated seismic signal to the deflectionplates of said cathode-ray tube and simultaneously applying to a controlelectrode of said cathode-ray tube a signal representative of said trainwaves to unblank said cathode-ray beam when said modulated seismicsignal is less than a predetermined value with said train of pulseslying between a predetermined area to form variable area peaks betweensinusoidal valleys and recording the displayed beam reflection on saidphotographic medium.

5. Method in accordance with claim 2 wherein lateral deflection of saidcathode-ray tube is confined to a predetermined width so that the top ofeach variable area peaks does not overlie the area of an adjacentseismic trace, when multiple traces are recorded side-by-side on thephotographic record.

6. The improved method of photographically recording lowamplitudeportions of a variable area display of a seismic trace instantaneouslyapplied by an electron beam to the face of a cathode-ray tube whichcomprises a. generating a first electrical signal having an invertedsawtooth waveform and having a frequency several times the highestfrequency of the information portion of a seismic trace, b. generatinganother electrical signal having a sinusoidal waveform corresponding tothe information portion of a field-generated seismic trace, 0.modulating said other electrical signal with said first electricalsignal to produce a composite signal, d. applying said compositedelectrical signal to the electron beam deflection element of acathode-ray tube. e. simultaneously applying to an intensity controlelement of said cathode-ray tube a blanking signal corresponding to apredetermined amplitude corresponding to the initial portion of eachinverted peak portions of said first electrical signal, and f.photographically recording the face of said cathode-ray tube withsimultaneous relative movement in at least one direction between thephotographic medium and said cathode-ray tube face, while saidcathode-ray electron beam is being controlled in response to saidcomposite signal and said blanking signals, whereby the recordedinverted sawtooth waveform is relatively uniform in line width anddensity at low amplitudes of the seismic signals to print a uniformlydense variable area on said photographic recording medium representingsaid seismic trace.

1. The method of plotting a generally sinusoidal voltage trace so thatthe area between said trace and its mean line is shaded on one side ofsaid means line by means of a sawtooth wave of substantially higherfrequency than that of said trace, comprising: a. generating a sawtoothvoltage of amplitude greater than the voltage difference between theexpected extrema of said trace and said mean line of said trace, b.applying at least one of the operations consisting of inversion andtranslation to said sawtooth voltage to cause its points of transitionfrom relatively slow amplitude change to relatively fast amplitudechange to be at a predetermined voltage relative to said generallysinusoidal trace, and to cause its points of transition from relativelyfast amplitude change to relatively slow amplitude change to be beyondsaid expected extrema, so that during the shading of said generallysinusoidal trace, said points of transition from relatively fastamplitude change to relatively slow amplitude change are not plotted insaid shaded area.
 2. The method in accordance with claim 1 in which saidpredetermined voltage of transition points from relatively slowamplitude change to relatively fast amplitude is the mean voltage ofsaid generaLly sinusoidal trace.
 3. The method of plotting a singleseismic trace in the form of a combined wiggle trace-variable areadisplay which comprises generating an electrical signal having aninverted sawtooth waveform, combining a seismic signal of generallysinusoidal waveform with a constant amplitude electrical signal and saidinverted sawtooth waveform to form a composite signal, modulating atleast one set of deflection plates of a cathode-ray tube in response tosaid composite signal, blanking the electron beam of said cathode-raytube impinging on the face of said cathode-ray tube when said invertedsawtooth wave contribution to said composite signal exceeds apredetermined value, and photographically recording the position of thecathode ray beam on the face of said cathode ray tube in accordance withthe time variations in the instantaneous amplitude of said compositesignal during the unblanking of said cathode-ray beam.
 4. The method ofrecording a seismic trace as a variable area display having sharpdefinition at low amplitudes which comprises applying to a photographicrecording medium the instantaneous output of an electron beam displayedon the face of a cathode-ray tube, said electron beam being under thecombined control of a sawtooth waveform having a frequency substantiallyhigher than the highest frequency of the seismic signal to be displayedand the seismic signal waveform, the improvement comprising invertingsaid sawtooth waveform, biasing said inverted waveform to generate awaveform emphasizing substantially only the peak portion of eachsawtooth wave, modulating at least a part of each cycle of said seismicsignal in response to a train of said biased and inverted waves,applying said modulated seismic signal to the deflection plates of saidcathode-ray tube and simultaneously applying to a control electrode ofsaid cathode-ray tube a signal representative of said train waves tounblank said cathode-ray beam when said modulated seismic signal is lessthan a predetermined value with said train of pulses lying between apredetermined area to form variable area peaks between sinusoidalvalleys and recording the displayed beam reflection on said photographicmedium.
 5. Method in accordance with claim 2 wherein lateral deflectionof said cathode-ray tube is confined to a predetermined width so thatthe top of each variable area peaks does not overlie the area of anadjacent seismic trace, when multiple traces are recorded side-by-sideon the photographic record.
 6. The improved method of photographicallyrecording low-amplitude portions of a variable area display of a seismictrace instantaneously applied by an electron beam to the face of acathode-ray tube which comprises a. generating a first electrical signalhaving an inverted sawtooth waveform and having a frequency severaltimes the highest frequency of the information portion of a seismictrace, b. generating another electrical signal having a sinusoidalwaveform corresponding to the information portion of a field-generatedseismic trace, c. modulating said other electrical signal with saidfirst electrical signal to produce a composite signal, d. applying saidcomposited electrical signal to the electron beam deflection element ofa cathode-ray tube, e. simultaneously applying to an intensity controlelement of said cathode-ray tube a blanking signal corresponding to apredetermined amplitude corresponding to the initial portion of eachinverted peak portions of said first electrical signal, and f.photographically recording the face of said cathode-ray tube withsimultaneous relative movement in at least one direction between thephotographic medium and said cathode-ray tube face, while saidcathode-ray electron beam is being controlled in response to saidcomposite signal and said blanking signals, whereby the recordedinverted sawtooth waveform is relatively uniform in line width anddensity at low amplitudes of the seismic signals to print a unIformlydense variable area on said photographic recording medium representingsaid seismic trace.